The present invention relates to a ceramic green sheet and a method for producing a ceramic substrate. The ceramic substrate is thin and hardly has surface defects, and it is used as a substrate for an electric circuit, a substrate for a multilayer circuit, a diaphragm, an elastic plate, etc.
Japanese Patent Laid-Open 55-134991 discloses a method for producing a thin ceramic green sheet or substrate. In the method, a ceramic powder, an organic binder, an organic solvent and the like are mixed together so as to obtain a ceramic slurry. As shown in FIG. 6, the slurry 10 is subjected to filtering by a mesh net 60 in a filter container 50 using a suction bottle 70 and a vacuum tube 80 so as to remove aggregates or the like in the ceramic slurry and produce a filtered slurry 90.
There has recently been a demand for a particularly thin ceramic substrate or ceramic green sheet having a thickness of, for example, at most 30 .mu.m. However, suitable properties of ceramic particles constituting such a ceramic substrate or a green sheet has been unknown. Upon this, attention has been paid to the size of the ceramic particles so as to make the particles finer or the like using the conventional technique for ceramics. However, it has been difficult to make a product which hardly has surface defects, and many products had to be disposed of as defective goods.
A ceramic slurry is prepared by mixing a ceramic powder with an organic binder and an organic solvent. Usually, the ceramic slurry inevitably contains coarse aggregated particles, dry coagulated grains, reaggregates, fine air bubbles, etc, besides impurities. The coarse aggregated particles and the like are imminent as defects in a ceramic green sheet and a ceramic substrate made from such a slurry. When the ceramic substrate is thin, such defects are prone to be actualized as a deterioration in the surface smoothness or the strength, occurrence of cracks, a deterioration in the dimensional stability, the electric insulation, the airtightness, etc. When a reverse roll coater method is employed, molding is conducted by a transferring process. Therefore, if the aforementioned coarse aggregated particles or the like are present in a ceramic slurry, a ceramic green sheet gains fatal defects on the surface. There has been a manufacturing problem of improving, particularly, a thin ceramic substrate having a thickness of at most 30 .mu.m, which is used as a substrate for an electric circuit, a substrate for multilayer circuit, a diaphragm, an elastic plate, etc.
Japanese Patent Laid-Open 55-134991 disclosed a method for removing coarse aggregated particles and the like, in which a net of 100-500 mesh is used for filtering a slurry. However, even a net of 500 mesh (25 .mu.m between each wire) cannot remove the coarse aggregated particles or the like of 25 .mu.m or less. To make the finer mesh is very difficult, and it causes clogging for a short period of time. Therefore, such a fine mesh was not put to practical use at all. Further, there are some other problems in the aforementioned method, i.e., because suction filtering is employed, an organic solvent in the ceramic slurry is prone to evaporate, which causes occurrence of undesirable air bubbles and partially causes occurrence of coagulated dry substance.